Synergistic effect of rice husk addition on hydrothermal treatment of sewage sludge: Fate and environmental risk of heavy metals

•HTT with RH addition was used for sludge stabilization and HMs immobilization.•RH addition and HTT had a synergistic effect on reducing the risk of HMs in sludge.•All the tested HMs showed no toxicity at a RH/sludge ratio of 1/1.75. Hydrothermal treatment (HTT) at 200°C was applied to immobilize heavy metals (HMs) and the effect of rice husk (RH) addition was investigated based on total HMs concentration, fractionation and leaching tests. The results indicated that a synergistic effect of RH addition and HTT could be achieved on reducing the risk of HMs from medium and low risk to no risk. Metals were redistributed and transformed from weakly bounded state to stable state during the HTT process under RH addition. Notably at a RH/sludge ratio of 1/1.75 (d.w.), all the HMs showed no eco-toxicity and no leaching toxicity, with the concentrations of leachable Cr, Ni, Cu and Cd decreased by 17%, 89%, 95% and 93%, respectively. This synergistic effect of RH addition and HTT on the risk reduction of HMs implies that HTT process with RH addition could be a promising and safe disposal technology for sewage sludge treatment in practice.